Aluminium is produced by electrolytic reduction of alumina dissolved in an electrolyte. Reduction results from the circulation of electrical current between one or more anodes and a cathode arranged in an electrolytic cell. Nowadays, Hall-Héroult aluminium reduction cells are operated at high current intensities often exceeding several hundred thousand amps.
Aluminium producers aim at increasing the current efficiency of the electrolysis cells and at decreasing the specific energy consumption of the same so as to reduce the operating costs of the aluminium reduction plants. The specific energy consumption of a cell, which is usually expressed in kWh/t, is equal to the energy consumed by a cell to produce one tonne of aluminium.
For that purpose, the aluminium producers seek ways to reduce the various electrical voltage drops that develop across an electrolytic cell and make the current distribution more uniform within the cell. Several patents have focused on a reduction in the cathode voltage drop Uc while often aiming at making the current flow more uniform over the surface of the cathodes. In particular, it is known that the cathode voltage drop Uc can be reduced by using composite collector bars including a steel part and a part made of a metal with an electrical conductivity higher than steel, usually copper.
French patent application No. FR 1 161 632 and U.S. Pat. No. 2,846,388 (Pechiney) describe electrolysis cells comprising copper plates that are adjacent the sides of the collector bars and extend all the way to the external end of the bars. Such arrangements are conducive to high thermal losses from the cells owing to the close proximity between the copper plate(s) and the aluminium busbars connected thereto.
U.S. Pat. No. 3,551,319 (Kaiser) describes an electrolysis cell comprising collector bars with a groove on their lower side and a copper conductor inserted within the grooves. U.S. Pat. No. 5,976,333 (Pate) describes arrangements wherein a copper conductor is inserted within a tubular collector bar. In both cases, the copper conductors are directly connected to the busbars. Such arrangements are also conducive to high thermal losses from the cell.
International application WO 02/42525 (Servico) describes arrangements wherein the copper conductor is encapsulated within the collector bar. International applications WO 01/63014 (Comalco) and WO 01/27353 (Alcoa) describe arrangements wherein copper conductors are inserted within the collector bars and separated from the connection means by a steel spacer in order to reduce the thermal losses of the cell. International patent application WO 2004/031452 (Alcan) and International patent application WO 2005/098093 (Aluminium Pechiney) describe arrangements comprising a copper insert and a varying sealing area between the collector bar and the carbonaceous block in order to improve the current distribution along the block. However, arrangements comprising inserts are quite difficult and expensive to make. Moreover, such designs make it difficult to significantly decrease the size of the collector bars.
Therefore the applicant addressed the issue of finding industrially acceptable solutions to the drawbacks of prior art, and particularly to the problem of specific energy consumption.